Clinical Investigations And Tests Of Parathyroid Functions: Serum Tests
Serum Analysis In The Laboratory
These tests include serum calcium, phosphorus, alkaline phosphate, parathyroid hormone, vitamin D and electrolytes. Other tests such as the suppression test are also discussed.
Serum calcium: In normal adults, the total serum calcium level is maintained between 9 and 11mg/dl, regardless of the intake. As age advances, serum calcium level gradually falls in men but it increases in women. Several factors affect serum calcium level. These are serum protein concentration, pH of blood, postural variations and the actions of PTH and vitamin D. While interpreting serum calcium values, all these factors have to be taken into account. Ionised calcium levels in plasma usually range from 4-5mg/dl, while plasma usually range from 4 to 5mg/dl. While collecting blood for calcium estimation venous stasis should be avoided.
Serum Phosphorus: In adults, the level of serum phosphate ranges from 2.5- 4.8 mg/dl. The levels are higher in children (0.5+1.6 mg/dl). Serum phosphate levels shows marked diurnal variation. Several factors such as serum protein levels, calcium, renal function, age, sex and diets rich in carbohydrates influence serum phosphate level. Blood has to be collected in the fasting state for phosphorus estimation. In hyperparathyroidism, serum phosphorus is low and vice versa.
Serum alkaline phosphatase: Total serum alkaline phosphatase or bone alkaline phosphatase reflects bone turnover in metabolic bone diseases. The normal adult value is 30-110 international units/liter (3- 13KA units/dl).
Serum parathyroid hormone: PTH can be quantified by radioimmunoassay (RIA). In hyperparathyroidism PTH is elevated and in hypoparathyroidism it is reduced. Presence of high levels of PTH along with high calcium levels suggest abnormality of control mechanisms. Elevated PTH levels in the presence of low calcium levels suggest pseudohypoparathyroidism. In magnesium deficiency PTH levels are low.
Serum Vitamin D: In healthy subjects, total vitamin D level is 35.0+3.4 ng/ml, 25 OHD3 is 28.5+2.0 ng/ml and 1,25 (OH)2D3 is 35.0+3 ng/ml. In hypoparathyroidism and chronic renal failure, circulating levels of 25 OHD3 are low. Dietary intake of calcium and phosphate also influences serum vitamin D levels.
Serum electrolytes: Since PTH inhibits the tubular absorption of bicarbonate, it may lead to renal tubular acidosis. The hypercalcemia of primary hyperparathyroidism is often associated with hyperchloremic acidosis. This feature helps to distinguish primary hyper-parathyroidism from hypercalcemia occurring in bony metastases or hypervitaminosis D in which there is also mild alkalosis.
Calcium Infusion Test
Suppression tests: Tests to determine the suppressibility of PTH are employed to differentiate various hypercalcemic states from hyper-parathyroidism.
Calcium Infusion test: Calcium infusion suppresses normal parathyroid, but not autonomouse parathyroid glands. Normally PTH level falls at the end of calcium infusion (15 mg/Kg body weight given over 4 hours and remains low for 12 hours. Urinary phosphate excretion falls by more than 30% during the infusion.
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